1. Field of the Invention
The present invention relates, generally, to a method of manufacturing a laminate including a base film and a metal foil for use in fabrication of a flexible printed circuit board (FPCB), and, more particularly, to a method of manufacturing a laminate for an FPCB, which comprises surface treating a base film to have excellent adhesive strength, heat resistance and dimensional stability.
2. Description of the Related Art
According to a recent tendency toward light weight, miniaturization, and high functionality of electronic products, PCBs have been increasingly used. Of the PCBs, in particular, an FPCB is advantageous because a circuit can be effectively formed even in a narrow space due to the use of a flexible and thin material, and the demand therefor is rapidly increasing.
In general, the FPCB includes a base film made of polyimide, polyester, or polyparavinyl acid, and a conductive metal foil made of copper, aluminum, iron, or nickel. As the base film, a polyimide film is mainly used thanks to its excellent thermal, electrical and mechanical properties.
Known as a conventional FPCB, a three-layered substrate includes a polyimide film and a copper foil bonded onto the polyimide film using an adhesive. Upon formation of a pattern, the three-layered substrate should undergo a heating process and a wet chemical treating process (etching, plating, developing, soldering, etc.). In this case, dimensional stability is decreased due to the difference in thermal expansion coefficient between the adhesive and the copper foil and between the adhesive and the polyimide film. In addition, the adhesive strength is decreased, attributed to the chemical treatment.
Electronic products, in particular, display devices, such as mobile phones and LCDs, are structured to be more complicated and denser. Simultaneously, the number of driver ICs functioning to drive devices and their degree of integration are further increasing. Thus, there is the need for a high density circuit pattern. However, since the three-layered substrate may cause the above problems due to the use of the adhesive, it is unsuitable for use in a high density circuit pattern.
To solve the above problems, thorough attempts have been made to manufacture a two-layered substrate for an FPCB having a metal layer formed by deposition, sputtering, ion plating, or copper plating, without the use of an adhesive.
Typically, the two-layered substrate for an FPCB may be manufactured by the following two techniques. That is, in the casting technique, polyimide liquid is applied on a metal layer, dried, cured and then processed into a film. In addition, the plating technique includes surface treating a polyimide film to realize high adhesive strength, coating the surface treated polyimide film with metal in a vacuum, and then conducting electrical plating using the above metal layer as a conductive layer, thereby forming a metal foil.
In the two-layered substrate without the use of the adhesive, the problem of low dimensional accuracy due to the difference in thermal expansion coefficient between the adhesive and the metal foil and between the adhesive and the polyimide film, as in the three-layered substrate, may be overcome. However, the adhesive strength between the metal foil and the polyimide film is remarkably weakened.
Hence, with the aim of improving the adhesion between the film and the metal to be deposited, a surface treated polyimide film may be used, or the surface of a polyimide film may be treated before deposition. In this way, the surface treatment of the polyimide film is carried out to wash the film as well as improve the adhesion between the polyimide film and the deposited metal.
The surface treatment process for the film includes mechanical treatment, such as blasting, hair line treatment and embossing, physicochemical treatment such as corona discharge and plasma treatment, or chemical solution treatment using a solvent, acid or alkali. Of these treatment processes, oxygen plasma treatment in a vacuum is frequently used in consideration of continuity to the subsequent procedures, productivity, contamination resistance, adhesion, and film degradation. The oxygen plasma treatment may be continuously performed while the film is transferred, followed by vacuum deposition, or may be independently performed.
Although the plasma treatment is conducted for two purposes, that is, washing and good adhesion, the adhesive strength and heat resistance are not increased as high as desired.
Further, the surface treatment of the base film by plasma is conducted at a rate of about 1 m/min. Therefore, an increase in the above treatment rate remains a task to be accomplished, to realize high productivity.